Microelectronic and micromechanical dies are often packaged to protect the die from an external environment and to provide connections to a PCB (printed circuit board), to a socket, to another package, or to some other interface. PCBs and sockets are typically made of a less expensive material that is not able to provide connections that are as close together as those of the die. A silicon or gallium arsenide die may have connection pads that are no more than 1 or 2 μm apart, while a PCB may have a structure that is able to provide connections that are no closer than 100 μm apart. The package typically has a substrate that connects the much closer connections of the die on one side and spreads them apart to form connections that are much farther apart on the other side.
Product architectures now contain several very different functions, such as CPU logic, graphics functions, cache memory, radio signaling, and other system functions to create integrated SOC (System on Chip) designs. The SOC tries to provide as many of these different functions as possible on a single chip. This lowers the product's design complexity and component count, simplifying manufacture, and reducing cost. However, depending on how many functions are combined, the single chip may be difficult to manufacture or limited in its performance.
Multichip packages allow each die to be built using the best or lowest cost manufacturing process and materials. The dies may then be combined into a single package, easing the assembly and design of the final product. Since the dies are often much smaller than the package, a multichip package may not be very much larger than a package for a single SOC. To further improve performance, a multichip package will have chip to chip interconnections inside the package. Wire leads and special substrates inside the package are used to accomplish these connections. For silicon dies, the special substrates are typically silicon substrates because silicon has the same CTE (Coefficient of Thermal Expansion) as the dies and because it can provide connection points which are as close together as those on the silicon die.
Because the temperatures inside the package can change from very cold when the end device is turned off and in a cold place to very hot when the package is powered up and running at maximum speed, the dies will expand and contract with temperature. If the connections are not flexible like wires or if the substrate expands and contracts at a different rate, then the connections may be broken.